Fiber Optics Dispersion and Loss Mechanisms - Loughborough University

Topics to be covered  Planar waveguides – mirror and dielectric waveguides, number of modes, field distribution, phase and group velocity  Fibre optics (circular waveguides) – fibre types, number of modes, acceptance angle, numerical aperture  Dispersion – material, modal  Loss mechanisms –...

Topics to be covered  Planar waveguides – mirror and dielectric waveguides, number of modes, field distribution, phase and group velocity  Fibre optics (circular waveguides) – fibre types, number of modes, acceptance angle, numerical aperture  Dispersion – material, modal  Loss mechanisms – absorption, scattering, bending  Definition  The broadening of a pulse of light as it propagates down an optical fibre is called dispersion  Two sources of dispersion in optical fibres:  Modal  Material  Dispersion limits the bandwidth of the fibre because broadening pulses eventually merge into one another making it impossible to distinguish between separate pulses  In fast data transfer links, dispersion must be minimised  In sensors, it can be useful to detect changes  Modal dispersion From Fundamentals of Photonics  Modal dispersion occurs in multimode fibres  Each mode has a different mode velocity  Rays that zigzag more take longer to arrive  Single pulse -> M pulses  Thus, the pulse energy is spread out in time – the pulse is dispersed  Modal dispersion - 2  Fibre of length L, the delay between the fastest and slowest mode στ (in seconds) is given by:  Single-mode fibre has no modal dispersion  Alleviated in multimode fibres by using graded-index fibre (GRIN)  Modal dispersion - 3 From Photonics and Lasers: An Introduction  Equalizes the different velocities of the modes and so reduces modal dispersion  Modes travelling close to the cladding travel faster because the decreasing refractive index  Axial modes travel slower  Material dispersion  Refractive index function of wavelength  Since a pulse of light is made up of a number of different frequency waves, the different frequencies will travel at different velocities causing the pulse to spread in time  n1 in fused silica has a higher variability at the lower end of the visible spectrum. At communication bands (1300nm and 1550nm), n1 variation is smaller  Summary  Three types of dispersion that occur in fibre optic cables have been identified  Modal - varying propagation path lengths of different modes  Material – variation of fibre’s refractive index with wavelength  Topics to be covered  Planar waveguides – mirror and dielectric waveguides, number of modes, field distribution  Fibre optics (circular waveguides) – fibre types, number of modes, acceptance angle, numerical aperture  Dispersion – material, modal  Loss mechanisms – absorption, scattering, bending  Introduction  Attenuation reduces the optical power received at the destination  Repeaters/amplifiers are employed periodically to boost the signal strength  There are three fundamental loss mechanisms:  Absorption  Scattering  Bending  Cannot be completely eliminated from the system  Absorption  Absorption is a basic property of the material used in the fibre optic cable  A photon travelling in a fibre can give its energy to an atom or a molecule  If the electron or molecule returns to its original energy state without reproducing a photon, one photon is lost and thus the total number of photons is reduced  The photon energy is lost in the material as heat  We define the attenuation coefficient to describe the power loss in a fibre  Attenuation Coefficient  Intensity of a beam of light propagating in a fibre optic cable decreases exponentially with increasing distance  The attenuation coefficient determines how quickly power is lost into the material, and is measured in m-1  Derived from the Beer-Lambert Law which relates the input power to the output power  The loss in a fibre can also be given in terms of dB: ( ) 𝑃 𝑖𝑛 𝑑𝐵 𝐿𝑜𝑠𝑠=10 𝑙𝑜𝑔10 =10 𝑙𝑜𝑔10 ( 𝑒 𝛼 𝐿 )=10 𝛼 𝐿. 𝑙𝑜𝑔10 𝑒=4.34 𝛼 𝐿 𝑃 𝑜𝑢𝑡 Attenuation Coefficient 𝟏 𝒆𝟎. 𝟓 𝟏 𝒆𝟏 𝟏 𝒆𝟐 𝟏 𝒆𝟑  Absorption - 2  Fused silica has two strong absorption bands  In the mid-infrared range, photons have sufficient energy to cause vibrational transitions in the material  In the ultra-violet range, photon energy is sufficient to cause electronic and molecular transitions  The tails of these two bands form a low absorption window in the near-infrared region  Standard communications bands are located in this window (1300nm and 1550nm)  Absorption - 2 From Fundamentals of Photonics  Attenuation coefficient of silica versus wavelength  Scattering  Fused silica is a non-crystalline material and as such will have imperfections in its molecular structure  The imperfections are caused by the random motion of glass before it is cooled into its final shape  These imperfections cause stationary fluctuations in the refractive index  Photons incident on these fluctuations are deflected (or scattered) in different directions  If the condition for total internal reflection or the Maxwell’s boundary conditions are no longer met, the photons will exit the fibre core  Photons do not loose energy nor change wavelength when scattered  This type of scattering is called Rayleigh Scattering  Rayleigh Scattering  Rayleigh scattering occurs when the scattering particle is much smaller than the wavelength of the light  Attenuation caused by Rayleigh scattering is proportional to , which means short wavelength light is scattered much more strongly than longer wavelengths  Less attenuation in the infrared region compared to ultraviolet when considering light propagating in a fibre optic cable  This is yet another reason why the communications bands are located in the near-infrared region  The theoretical lower limit for Rayleigh attenuation in silica glass is 0.15dB/km.  Typical attenuation spectrum From Fundamentals of Photonics  Attenuation coefficient of silica versus wavelength  Local minimum at 1300nm (α ≈ 0.28dB/km)  Absolute minimum at 1550nm (α ≈ 0.15dB/km)  Bending losses  Light can escape from a fibre if the fibre is bent  A ray of light which would normally undergo total internal reflection at the core- cladding boundary may no longer do so because of the change in the fibre geometry  These rays will be refracted into the cladding and will be lost The bend radius at which an axial  This results in a reduction in ray would exit the fibre is given by the the received optical power approximation:  Loss caused by cable bending is called bending loss  Summary  Optical power input into a fibre optic cable is attenuated by three main mechanisms  Absorption – photons with sufficient energy can excite electrons and molecules in the material, thus losing their energy as heat  Scattering – imperfections in the material structure causes photos to be scattered out of the core without losing their energy  Bending – light that would normally be totally internally reflected escapes out of the core because this condition is no longer satisfied